Application developers are the customers of an API. Success is measured by how quickly app developers enjoy success using your API in their applications. And rapid adoption of an API is all about design.

Web API Design will help you make design choices from the application developer’s point of view so that the benefits of proven design principles and best practices will make your initiative a success.

Affordance - the key design principle

Start with errors

Versioning is essential

Pagination and partial response

Tips for search

Authentication

Chatty APIs

Complement with an SDK

The API Facade Pattern

The e-book Web API Design from @Apigee is a joy to read, thanks Brian @landlessness